The mechanism of insulin's regulation of various metabolic pathways remains poorly understood. Insulin acts as a physiologic antagonist of cyclic AMP in many tissues; therefore, it has been hypothesized that insulin may act through calcium as a second messenger. With the discovery of calmodulin, the mechanism of hormonal regulation via activation of the Ca++/calmodulin complex has begun to be recognized. There is further evidence which suggests that the Ca++/calmodulin complex can activate or inactivate certain kinases or phosphatases which appear to be regulated by insulin. We propose to address the hypothesis that insulin acts through a Ca++/calmodulin dependent phosphatase, calcineurin, to modulate the cAMP signal in cells. We will do this by developing an antibody to hepatic calcineurin and studying this enzyme in detail in isolated rat hepatocytes or hepatoma cells variously treated with insulin, glucagon or Beta agonists. These studies will be extended to study the enzyme in livers of streptozotocin-induced diabetic rats and db/db mice. We hope that these studies will further our understanding of insulin action in liver and the pathophysiology of diabetes.